Case hardened stainless steel oilfield tool

ABSTRACT

An oilfield tool comprising a body of high chromium, low carbon steel and a high hardness surface having a supersaturated level of carbon. The body may be manufactured from a Super 13Cr type stainless steel. The tool may be carburized to a supersaturated level providing a surface hardness of about 55 HRC to 62 HRC and have an effective case depth of at least 0.020 inch at which depth the hardness is 50 HRC. The tool body is heat treated to a body hardness of about 39 HRC. A surface of the tool may be surface coated, e.g. nitrided or borided. The tool may be an expansion cone and may be a variable diameter expansion cone.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

FIELD OF THE INVENTION

The present invention relates to tools used in the oilfield and more particularly to case hardened stainless steel tools for use in boreholes.

BACKGROUND OF THE INVENTION

It has become common to use open hole completions in oil and gas wells. In these wells, standard casing is cemented only into upper portions of the well, but not through the producing zones. Tubing is then run from the bottom of the cased portion of the well down through the various production zones. Modern techniques include the use of expandable solid or perforated tubing and/or expandable sand screens. These types of tubular elements may be run into uncased boreholes and expanded after they are in position. Expansion may be accomplished by pulling or pushing an expansion tool or cone through the tubular members. An expansion cone must be able to withstand very high forces as it is pushed or pulled through tubing in the expansion process. The cone must be made of high strength material. The cone preferably has a hard surface to resist wear and reduce friction. However, it has been found that as the hardness of cones is increased to reduce wear and friction, the cones become more brittle, or less tough, and are subject to breaking in actual field operations.

SUMMARY OF THE INVENTION

The present invention provides an oilfield tool having a body portion made of high chromium low carbon steel alloy and having a surface which has been supersaturated with carbon to produce a very hard and wear resistant case.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a variable diameter expansion tool according to an embodiment of the present invention.

FIGS. 2, 3 and 4 are photomicrographs of carburized steel samples according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, there is illustrated an oilfield tool 10 according to an embodiment of the present invention. In this embodiment, the tool 10 is a variable diameter expansion tool suitable for expanding expandable tubing in a borehole. The tool 10 has at least one conical or wedge shaped portion 12 which performs a tubing expansion function when the tool 10 is forced through expandable tubing or other expandable elements. Due to this conical shaped portion 12, this type of expansion tool is commonly referred to as an expansion cone. Many prior art expansion tools comprise fixed diameter cones. The FIG. 1 cone provides a variable expansion diameter. The variable diameter is achieved by providing longitudinal expansion slots 14 which allow the cone section 12 to expand radially. The tool 10 may be run into the borehole with a minimum diameter, and then expanded to a working diameter for expanding tubing. With proper controls, the diameter of the cone 10 may be adjusted while expanding tubing to compensate for variable borehole diameters.

Expansion cones are subjected to extreme frictional loads as they are used to expand tubing. The frictional loads may cause rapid wear of the cone, increase the force required to expand tubing, and may cause extreme heating of the cone. In an effort to reduce wear and friction, expansion cones have been made of very hard steel, for example D2 tool steel. D2 steel is a high chromium, i.e. 15% to 17% chromium, and high carbon, about 1.65% carbon, steel. The microstructure of D2 steel is pro-eutectic, that is, it is supersaturated in carbon. The excess carbon combines with chromium to form very hard chromium carbides. When surrounded by a very high strength, martensitic microstructure, the chromium carbides provide good abrasion and wear resistance to the D2 steel. The hardness of D2 steel is in the range of 55 HRC to 62 HRC. However, D2 steel and other high chromium, high carbon steels of similar hardness are very brittle. Oilfield tools made from such materials are susceptible to brittle fracture from the high stress and impact loads to which oilfield tools are commonly exposed when used in a borehole. For example, expansion cones, especially variable diameter expansion cones, made from such materials are susceptible to brittle fracture from the high stress and impact loads experienced when expansion tools are used to expand tubing in boreholes.

Variable expansion tools like the FIG. 1 cone made of D2 tool steel have experienced failures in actual use in oil wells. Analysis of the failed cones confirmed that brittle failure occurred. As a result, other materials with better ductility and toughness were considered for use as expansion tools. A steel alloy used in various oilfield applications because of its combination of toughness and corrosion resistance is a low-carbon martensitic stainless steel known by the trade name Super 13Cr. Versions of this steel are made by various mills and are used in a number of oilfield applications where tough corrosion resistant parts are needed. While the various versions of Super 13Cr may differ somewhat in composition, they all have about 13% chromium, about 0.03% carbon and no more than 0.05% carbon, about 5% nickel and about 2% molybdenum. The low carbon content limits the attainable hardness to about 39 HRC.

While the toughness of Super 13Cr and similar alloys is highly desirable in a variable expansion tool, the attainable hardness is too low for use as an expansion tool and many other oilfield applications. In an effort to reduce wear and friction, various surface treatments such as boriding and nitriding were considered and tested. Such coatings can provide surface hardness in the range of 50 HRC to 70 HRC which would be equivalent to or better than the hardness of D2 steel. However, such coatings are generally very thin, typically less than 0.010 inch, and subject to cracking, crushing, flaking, spalling or other failure, due in part to a sharp contrast in hardness between the coating and the supporting metal.

In order to have the toughness benefit of a Super 13Cr type steel and the surface hardness of a D2 type steel, it was determined that the surface carbon content of an expansion tool made of Super 13Cr type steel should be increased to supersaturated levels so that chromium carbides would precipitate at the desired surfaces. The surface of the cone could then have a composition and hardness about the same as D2 steel, but the body of the cone could have the desirable toughness of a Super 13Cr type steel.

Samples of Super 13Cr steel were treated by the following process. A gas carburizing process was used to carburize the samples at 1760° F. for ten hours at 1.3% carbon potential, followed by 4 hours at 1.1% carbon potential. The temperature was reduced to 1550° F. and held at 0.9% carbon potential for one hour. The samples were air cooled. After the carburizing process, the samples were hardened by heating to 1850° F. in a vacuum and quenching to room temperature by backfilling the vacuum chamber with nitrogen. The samples were then tempered at 500° F. for one hour, cooled to at least 100° below zero degrees F. with nitrogen gas, tempered again at 500° F. for one hour, and finally air cooled to room temperature. Other heat treating processes, including other carburizing, hardening and tempering methods, may be used if desired. This process rendered a case hardness of 59 HRC on the surface, an effective case depth of about 0.040 inch at 50 HRC, total case depth of about 0.140 inch, and a core hardness of about 39 HRC. Core properties were 126,667 psi yield strength, 157,949 psi ultimate strength, 17% elongation, 61% reduction in area, and 123 foot pounds Charpy impact toughness at room temperature. These core properties are equivalent to uncarburized Super 13Cr type steel which has been given the same heat treatments. The core properties were measured on samples which received the same time-temperature cycles as the carburized samples, but which were coated with “no carbon” paint to prevent case hardening.

Measurements of the carbon taken at various depths reveal that the case carbon concentration varied from about 1.7% carbon at the surface to about 0.05% carbon at the core. The surface hardness of 59 HRC is in the normal range of D2 steel, i.e. 55 HRC to 62 HRC, which has a carbon content of about 1.65%. At the effective case depth of about 0.040 inch where the hardness measured 50 HRC, the carbon content was 1.4%. At the total case depth of about 0.140 inch, the core hardness of about 39 HRC is due to the original core carbon content of about 0.05% carbon or less.

FIGS. 2, 3, and 4 are photomicrographs of a carburized sample described above taken at a magnification of 650×. FIG. 2 is at the surface and shows evenly dispersed, fine carbides. This indicates a high or supersaturated carbon concentration at the surface. The saturation level in steel is normally considered to be about 0.7% carbon. Concentration levels above about 0.7% carbon are considered supersaturated. As noted above, the hardness measurements are consistent with tool steels which are supersaturated in carbon. FIG. 3 is taken at a depth of about 0.010 inch below the surface, where the carbon measured about 1.6%, and shows carbides mainly in networks. The hardness still measured 59 HRC, which is not attributed to the hardness of the carbides but rather to the high-carbon martensite in the surrounding matrix. From this point to a depth of about 0.060 inch, there is a gradual decrease in both the hardness and volume of carbides until the microstructure is below saturation carbon level. FIG. 4 is at a depth of about 0.080 inch below the surface, and shows a structure typical of lower carbon stainless steel, i.e. tempered martensite with a hardness of 41 HRC. The sample was treated with Marbles etch.

It has generally been believed that high chromium steels cannot be carburized due primarily to the high chromium content itself. However, the results achieved with the process described above show that Super 13Cr type steel can be treated to increase its surface hardness to the equivalent of D2 steel, while retaining the body toughness of Super 13Cr type steel.

A 6- 1/8 inch expansion cone 10 and an 8- 1/2 inch expansion cone 10 of FIG. 1 were made by forming the tools from Super 13Cr type steel and carburizing the tool by the process described above to form a hardened case on the surface of the tool. The cones may be formed from a cylinder of Super 13Cr type steel by machining. Alternatively the cones may be formed by forging, casting, or any other conventional method of forming a tool from a steel alloy. Each cone was tested by expanding 500 feet of tubing in a well. Neither cone suffered any mechanical failure. Neither showed noticeable wear.

While Super 13Cr type steel was used in the embodiments described herein, it should be apparent that other high chromium, low carbon steel alloys are suitable for use in the present invention. Such alloys may have from about 8% chromium to about 19% chromium, and preferably from about 11.5% chromium to about 18% chromium. The upper end of the chromium range, i.e. from about 13% chromium to about 19% chromium and preferably from about 13% chromium to about 18% chromium, may be preferred for retaining surface corrosion resistance because formation of chromium carbides at the surface reduces the free chromium content at the surface. Suitable alloys should have less than 0.1% carbon, preferably less than 0.05% carbon, and typically in the range of 0.02% to 0.03% carbon. Suitable alloys may have up to 7%, and preferably up to 5%, nickel, up to 3% molybdenum, up to 0.5% titanium and up to 0.5% vanadium. The toughness elements, i.e. nickel, molybdenum, titanium, and vanadium, may generally be interchangeable, but the total amount should be limited to no more than about 10% since higher percentages may slow carbon diffusion and may prevent the alloy from being martensitic. Martensitic alloys are preferred for the present invention. Any steel alloy meeting these composition limitations is considered to be a high chromium low carbon steel alloy for purposes of the present invention.

The processes described above resulted in an effective case depth of about 0.040 inch and a total case depth of about 0.080 to 0.140 inch. Effective case depth is the depth at which the hardness is 50 HRC and total case depth is the depth at which the hardness reaches average core hardness. It is preferred that the effective case depth be at least 0.020 inch and that there be a gradual transition in hardness from the surface down to the core hardness. Case depth is determined primarily by diffusion time and temperature. The fact that the carburizing process is diffusion based generally provides a desirable gradual transition to the core hardness.

After achieving good results with the gas carburizing process described above, another method of carburizing tools made of a high chromium low carbon steel alloy was tested. Several cones made of Super 13Cr type steel were carburized by a vacuum carburizing process provided by Surface Combustion, Inc. of Maumee, Ohio. In this process, the cones were carburized at 1750 degrees F. for five to six hours in 9.6 torr vacuum with 11 cc per minute flow of cyclohexane. The samples were then cooled to room temperature. After the carburizing process, the cones were hardened by heating to 1750° F. in a vacuum and quenching to room temperature by backfilling the vacuum chamber with nitrogen. After hardening, the cones were cooled to at least 1000 below zero degrees F. with nitrogen gas, tempered at 500 degrees F. for one hour, air cooled to room temperature, again tempered at 500 degrees F. for one hour, and finally air cooled to room temperature. Other heat treating processes, including other carburizing, hardening and tempering methods, may be used if desired. Cones subjected to this treatment achieved a surface hardness of 62 HRC, an effective case depth of about 0.02 inch at 50 HRC, and a total case depth of about 0.08 inch. This vacuum carburizing process may be a preferred carburizing method for making oilfield tools according to the present invention.

High chromium low carbon steel alloys are very temper resistant, i.e. little loss of strength occurs with temperature. As a result, they are suitable for other surface hardening treatments such as nitriding, boriding or other treatments recognized by those of skill in the art. Such treatments may further raise the surface hardness and may reduce the coefficient of friction. Such coatings may be more rugged when applied to the surface of a tool with a high surface hardness from the carburization, since no sudden hardness transition will occur.

The present invention provides oilfield tools with hard wear resistant and corrosion resistant surfaces and strong tough bodies. Many oilfield tools, especially completion tools, have similar requirements for surface hardness, toughness, and corrosion resistance. For example, cones and swages for tubular expansion tools; slips for packers, liner hangers, and similar devices that anchor in casing; shifting tools for operating down hole tools; seal surfaces such as packer bores or other sealing bores or outer surfaces; and, blast joints may all benefit from these properties. Such oilfield tools may be made from a high chromium, low carbon steel and carburized as taught herein to achieve an improved tool. Such tools may also be borided, nitrided or otherwise surface treated if desired.

While the present invention has been described with respect to a particular oilfield tool, particular alloys and methods of carburizing and other heat treatments, it is apparent that various changes may be made and various alternative materials may be used within the scope of the present invention as defined by the appended claims. 

1. An oilfield tool, comprising: a body comprising a high chromium low carbon steel alloy, and a case at least partially supersaturated with carbon.
 2. An oilfield tool according to claim 1, wherein the steel alloy comprises from about 8% chromium to about 19% chromium.
 3. An oilfield tool according to claim 1, wherein the steel alloy comprises from about 13% chromium to about 19% chromium.
 4. An oilfield tool according to claim 1, wherein the steel alloy comprises less than about 0.10% carbon.
 5. An oilfield tool according to claim 1, Wherein the steel alloy comprises a super 13Cr type stainless steel.
 6. An oilfield tool according to claim 1, wherein the steel alloy comprises less than about 0.05% carbon.
 7. An oilfield tool according to claim 1, wherein the case comprises from about 0.1% carbon to more that about 0.7% carbon.
 8. An oilfield tool according to claim 1, wherein the case comprises from about 0.1% carbon to about 1.6% carbon.
 9. An oilfield tool according to claim 1, wherein the case carbon saturation varies from supersaturated at the surface of the tool to about 0.1% or less adjacent the body of the tool.
 10. An oilfield tool according to claim 1, wherein the case carbon saturation varies from about 1.6% at the surface of the tool to about 0.1% or less adjacent the body of the tool.
 11. An oilfield tool according to claim 1, wherein the case has an effective case depth of from about 0.020 inch to about 0.040 inch.
 12. An oilfield tool according to claim 1, wherein the case has a total case depth of from about 0.080 inch to about 0.140 inch.
 13. An oilfield tool according to claim 1, wherein the steel alloy comprises a martensitic microstructure.
 14. An oilfield tool according to claim 1, further comprising a surface coating on at least one surface.
 15. An oilfield tool according to claim 14, wherein the surface coating is selected from a nitride and a boride.
 16. An oilfield tool according to claim 1, wherein the tool comprises a completion tool.
 17. An oilfield tool according to claim 1, wherein the tool comprises an expansion cone.
 18. A method of making an oilfield tool, comprising: forming the tool from a high chromium low carbon steel alloy, and carburizing the tool to produce a case at least partially supersaturated with carbon on at least one surface of the tool.
 19. A method according to claim 18, further comprising further heat treating the tool after carburizing.
 20. A method according to claim 19, wherein the further heat treating comprises multiple heating and cooling steps.
 21. A method according to claim 20, wherein one of the cooling steps is a sub-zero cooling step.
 22. A method according to claim 19, wherein further heat treating comprises hardening.
 23. A method according to claim 22, wherein further heat treating comprises tempering.
 24. A method according to claim 22, wherein further heat treating comprises tempering after hardening.
 25. A method according to claim 19, wherein further heat treating comprises tempering.
 26. A method according to claim 18, further comprising using a gas carburizing process.
 27. A method according to claim 18, further comprising using a vacuum carburizing process.
 28. A method according to claim 18, further comprising nitriding at least one surface of the tool.
 29. A method according to claim 28, wherein nitriding occurs after carburizing.
 30. A method according to claim 18, further comprising boriding at least one surface of the tool.
 31. A method according to claim 30, wherein boriding occurs after carburizing. 